The present invention relates to detection of a combustion state of an internal combustion engine (hereinafter simply referred to as an engine) and more particularly, to engine combustion state detecting apparatus and method capable of detecting a misfire in all running regions of the engine in which positive torque is generated, and to a recording medium which stores a program for execution of the detecting method.
A technique for measuring an engine speed by utilizing the relation between torque generated through combustion cycles of the engine and engine speed and detecting a running state of the engine to indirectly detect a misfire is described in, for example, Japanese Patent Application laid-open No. JP-A-58-51243. In this technique, engine speeds are detected at two ignition time points during one ignition cycle covering the preceding ignition and the present ignition, a change value of engine speed during the one ignition cycle is determined from a difference in engine speeds between the two ignition time points, change values of engine speed sequentially determined are operationally processed in statistical manner, and the presence or absence of a misfire in the engine is decided using results of the operational process.
The aforementioned method is effective to detect a misfire at relatively low engine speeds but production errors in pitch of a disk plate or a ring gear mounted for measurement of the engine speed greatly affect the detection at high engine speeds, making it difficult to detect a misfire. A countermeasure there-against is a method disclosed in JP-A-9-72241 by the present inventors, according to which an engine speed signal (time signal) is corrected by detecting a pitch error component during normal combustion.
The pitch error correcting method disclosed in the above gazette is effective when the engine is running at a high engine speed but at low engine speeds or under no load (idling), the accuracy of misfire detection tends to be degraded.
Incidentally, when a vehicle runs on a bad road or a rough road, the engine backwardly receives torque (negative torque) from driven wheels. Since the backward torque has an inconstant value, the engine undergoes a change in engine speed which is not attributable to the combustion state. When the vehicle carrying a misfire detecting apparatus based on the above pitch error correcting method runs on a bad road, negative torque affects the misfire detection.